In the refining of slags for the purposes of recovering valuable metal the process involves handling much of the slag which is worthless. However, a discrete portion consists of a mixture of free metal and non-metallic substance in percentages that could make it attractive to separate the metal from the slag with a proper process. One technique includes a rotating furnace mounted on a horizontal axis for heating the slag either internally or externally to treat either wet or dry slag, in an effort to separate the metal component. More particularly, the entrained metal and metal oxides differ in their density so that they separate when they are brought to a liquid state and agitated in the hot furnace. Thus separated, the unwanted slag portion can be decanted leaving the metal sought to be recovered or the metal can be poured off leaving the slag.
The foregoing relates to furnace processes which are popular. Those processes are unsatisfactory in that at least 5% of the metal remains trapped in the slag, and/or the heat required for the process is energy intensive thus being costly. In addition, in those processes which utilize the thermal energy associated with the exothermic oxidation of the metallic species such exothermic reactions destroy a significant proportion of the free aluminum metal when generating heat. Moreover, the operation of the furnace is expensive in terms of maintenance and power consumption.
Furthermore, the salts remain trapped in the slag, and these salts are necessary in the furnace refining process to act as a flux in protecting the metal from oxidation. It is, therefore, expensive to use the rotating furnace process in that 5% of the metal is not recoverable or recyclable, and disposal of the remaining slag containing salt presents problems. Furthermore, the use of such slags in land fill, in the manufacturing of pavement and concrete, and for fill in refractory or other similar cements is prevented.
Other techniques for the processing of salt containing furnace slags have been shown to exist and are related to leaching and washing processes that use water to extract soluble salts from the insoluble metals and their oxides. Such processes are known in connection with the treatment of slag. More particularly, the Papafingos et al U.S. Pat. No. 4,073,644 describes a method of processing aluminum slag by leaching wherein the slag is put in a rotating drum set about an axis which is elevated slightly from horizontal such that solubles go into a solution with water and the insolubles either remain suspended or sink, depending upon their particle size. The suspended insolubles in the slurry can thereby be removed from the low end of the rotating drum. Any remaining materials in the slurry can then be separated by a trap and filters. The remaining solution is distilled to remove the salt.
The problem with such a leaching process is that the water solvent has been found to react with the free metal and/or other trace impurities such as aluminum carbide and aluminum nitride in the slag, generating heat and resulting in the evolution of dangerous gases. Furthermore, these reactions are uncontrolled and powerful. More particularly, as much as 50,000 gallons of slag and solvent can be raised nearly 150.degree. F. in less than one hour causing the evolution of methane, hydrogen and ammonia and/or hydrogen sulfide. Such an uncontrolled reaction is unsafe in that fires will ensure which could destroy the leaching equipment and consume the recoverable metal. Moreover, the air quality around the process is totally unacceptable. The improvement of the present invention relates to a means by which the reaction in the leaching drum can be minimized and the severe chemical processes creating hazardous gases and resulting in increases in temperature can be controlled.